1. Field of the Invention
The present invention relates generally to the measurement of position or location by means of electromagnetic signaling and more particularly to position measurement utilizing near field signals in conjunction with a calibration process.
2. Related Art
Radio frequency (RF) techniques have been proposed to solve many ranging and position measuring problems in industry. For example, significant cost reduction is possible if inventories could be automatically tracked in a warehouse. Hospitals need to know the location of resources such as wheel chairs, gurneys, and diagnostic equipment for speedy retrieval when needed and for cost efficient operations. Hotels and resorts need to know the location of resources such as projectors, lawn mowers, golf carts, etc. Position information could inform a security system keeping track of inventories in a retail establishment and guarding against theft. Position information is critical to the national 911 system to enable first responders to know instantly the location of a call to 911 from a person in distress.
Accurate, affordable position information however has been elusive. A principal source of difficulty arises from the fact that typical environments are not ideal. Outdoors, typical environments contain objects such as trees, hills, buildings, cars and such that disrupt ideal planar uniform behavior. Similarly inside, objects such as walls, studs, pipes, desks, filing cabinets, and lights tend to attenuate or block signals as well as generate multi-path reflections. In both cases, real world environments have complicated behaviors that defy exact reliable predictions.
A variety of prior art seeks to overcome complicated propagation environments by mapping a signal characteristic corresponding to particular locations of interest. These techniques are sometimes collectively referred to as “RF fingerprinting.” The motivation behind these techniques is the hope that a sufficiently accurate map can be made to uniquely identify a particular transmit position in the same way a human fingerprint serves to uniquely identify a particular person.
One RF fingerprinting approach is to deploy a network of sensors throughout an area in which one desires to track personnel or assets. Received signal strengths at each sensor may be compared to calibration, reference or experimental data to determine which previously measured location yields the best fit to a currently received signal. Christ (U.S. Pat. No. 5,977,913) uses this technique to localize personnel and Gray et al (U.S. Pat. No. 6,674,403) use this technique to track wireless devices. However, positioning based on relative signal strength is notoriously inaccurate. Network signal strength measurements may serve to localize a transmitter to a particular zone, but usually require at least one sensor per zone. This often makes it uneconomical to achieve high precision positioning. Also, the propagation environment may change significantly based on the presence of people, goods, or other transient objects that may not have been present or may have been in different positions at the time a calibration was performed.
An alternate RF fingerprinting technique attempts to use multi-path signals arriving at an antenna array to localize a transmitter. Multipath signals arriving at the antenna array are compared to a database of calibrated multipath signal signatures and corresponding locations. The location whose calibrated signal signature best matches the measured signature is selected as the most likely transmitter location. Hilsenrath (U.S. Pat. No. 6,026,304) suggested this technique in conjunction with a system to localize cellular phone transmissions. More sophisticated techniques for signature matching were taught by Wax et al (U.S. Pat. Nos. 6,064,339; 6,104,344; 6,108,557; 6,112,095). These techniques may be used to make more economical assignments of cellular subscribers to base stations as taught by Grubeck et al (U.S. Pat. No. 6,154,657), or applied to CDMA systems as taught by Wax et al (U.S. Pat. No. 6,249,680). Furthermore, Wang et al (U.S. Pat. No. 6,282,426) teach using time of arrival signals and simulated ray tracing. All of these techniques rely on the hope that the multi-path environment will be sufficiently stable and static to be repeatable.
Chen et al (U.S. Pat. No. 6,496,701) teach a system in which the geographical location of a mobile terminal is identified by comparing characteristics such as pilot strength and chip offset from the mobile terminal with the same attributes for a variety of sub-cells and determining which sub-cell most closely matches the observed set of RF characteristics. Werb et al (U.S. Pat. No. 6,456,239) teach user selectable configuration packages in conjunction with a system for determining location of a tag using stored data. Moriya et al (U.S. Pat. No. 6,691,074) teach using accelerometers and Kalman filtering to supplement electromagnetic position measurements.
Finally, there is a body of prior art involving signals conveyed on a transmission line such as a telegraphy line or a power line. Edison (U.S. Pat. No. 162,633) taught an apparatus for duplex telegraphy in which direction of current yields one signal channel and increase or decrease of current yields another.
Thus, there is a need for a low cost method for range determination that may be used in complex RF propagation environments such as in and around buildings or over rough terrain and yet provide accurate, reliable results.